SemiconductorLasers

By Vidhya Moorthy

Objectives• Semiconductor materials• P N Junction Diode• History Of Semiconductor Lasers• Types Of Semiconductor Based On Emission• Principle Of Semiconductor Laser• Classification Of Semiconductor Laser• Homojunction Semiconductor Laser• Heterojunction Semiconductor Laser• Characteristics• Advantages • Applications

• Semiconductor materials are a combination of third group elements of the Periodic Table.• They have a resistance of 10^-

2.• These materials usually have a

‘valence band’ and ‘conduction band’ of electrons.• Normally there is no electrons

in the conduction band ; the valence band is completely filled with electrons.

SemiconductorMaterials

When a small amount of energy is given the electrons jump from valence band to conduction band to conduct current.

Conduction band

Valence band

ENERGY BAND

DIAGRAM

ENERGY

P N Junction Diode

• It consist of a junction of a P-type semiconductor and N-type semiconductor.

• P-type semiconductor : conduction through positive holes; doped with trivalent impurity.

• N-type semiconductor : conduction through negative electrons; doped with pentavalent impurity.

Types Of Semiconductor Based On Emission

Semiconductor materials are of two types based on emission ;• Direct Band Gap Semiconductor• Indirect Band Gap Semiconductor Direct Band Gap SemiconductorWhen the holes and electrons combine during the action of the diode; Photons are emitted.Example : Ga-As diodeIndirect Band Gap SemiconductorWhen the holes and electrons combine during the action of the diode; Heat Energy is emitted.Example : Ge, Si diodes

Direct Band Gap & Indirect Band Gap Diode

DIRECT BAND GAP DIODE

INDIRECT BAND GAP DIODE

History Of Semiconductor Lasers• The first laser diodes were

developed in the early 1960s • The device shown is an early

example. It would require very high current flow to maintain a population inversion, and due to the heat generated by the steady-state current, the device would be destroyed quickly.

• Nowadays technological innovations have made Diode lasers more powerful and strong.

Milestones Of Semiconductor Laser

Basov, Vul, Popov, Krokhin: 1957 first semiconductor laser proposal and development1961 first injection laser proposal (also Dumke 1962)Basov: Nobel prize 1964

(with Prokhorov and Townes

Innovations in Diode LaserLaser operatimg at room temperature

Principle Of Semiconductor Laser Recombination Radiation

When the P-N Junction diode is Forward Biased (i.e) the P end of the diode is connected to the positive terminal of the battery and the N end is connected to the negative terminal of the battery. The poles and electrons diffuse through the junction and combine with each other; meanwhile light radiations or photons are radiated. This is called Recombination RadiationThese emitted photons stimulate Other electrons & holes to recombine which Results in stimulated emission required forLasing Action.

Classification Of Semiconductor Laser

Semiconductor Laser

Homojunction Diode Laser

Heterojunction Diode Laser

Double Heterojunction

Diode Laser

Single Heterojunction

Diode Laser

Homojunction Semiconductor Laser

Homojunction diode lasers are those in which P end and N end of the diode are made of the same semiconductor material.Example : Ga As laser• They use Direct Band Gap Semi- conductor material.• P-N Junction act as the active medium.• The crystal is cut at a thickness of 0.5 mm• Applied voltage is given through metal contacts on both surfaces of the diode.• Pulse beam of laser of 8400 Å is produced

Working

FORWARD BIASED DIODE LASER

metal contact

Ga –As material on both ends

P end

N end Laser beam +

Energy Level Diagram : Homojunction

FREE ELECTRONS

Conduction Band energy barrier

stimulated emission Band gap (Eg)Energy laser (8400 Å ) FREE HOLES

Valence Band

Heterojunction Semiconductor Laser

Heterojunction Semiconductor lasers are those in which P end is made of one type of semiconductor material and the N end is made of another type of semiconductor materialExample : GaAIAs diode laserUse Direct Band Gap Semiconductor Consist of five layers namely• GaAs – p type• GaAIAs – p type• GaAs – p type (Active Medium)• GaAIAs – n type• GaAs – n typeThe end faces of the third layer is highly polished and perfectly

paralell to each other to reflect the laser beam ; one end is partially polished to release the continious beam.

Working

metal contactP end

N end Laser Beam

Ga As

GaAIAs

Energy Level Diagram : Heterojunction

Characteristics Of Semiconductor Laser

• Most SC lasers operate in 0.8 – 0.9 µm or 1 – 1.7 µm

spectral region• Wavelength of emission determined by the band

gap• Different SC materials used for different spectral

regions• 0.8 – 0.9 µm : Based on Gallium Arsenide• 1 – 1.7 µm : Based on Indium Phosphide (InP)• Pumping method : Direct Conversion• High power lasers usually (1 mV )

ComparisionHOMOJUNCTION DIODE

LASER

• P and N regions are made of the same diode material

• Active medium : Single crystal of PN Diode

• Pulse beam• Wavelength : 8300Å-

8500Å• Example : GaAs,InP.

HETEROJUNCTION DIODE LASER

• P and N regions are made of different diode material

• Active Medium : Third layer of p type material among the five layers

• Continuous beam• Wavelength : 8400 Å• Example :

GaAs/GaAIAs, InP/InAIP .

Advantages• They are light weighted and portable.• Battery supported ; easily replaceable • Capability of direct modulation into Gigahertz

region• Small size and low cost• Capability of Monolithic integration with electronic

circuitry• Direct Pumping with electronic circuitry• Compatibility with optical fibres

APPLICATIONS OF DIODE LASER

DISK READERS

INDUSTRIAL USAGE

PORTABLE LASERS

General Working Theory• The semiconductor materials have valence band V and conduction band C, the

energy level of conduction band is Eg (Eg>0) higher than that of valence band. To make things simple, we start our analysis supposing the temperature to be 0 K. It can be proved that the conclusions we draw under 0 K applies to normal temperatures.

• Under this assumption for nondegenerate semiconductor, initially the conduction band is completely empty and the valence band is completely filled. Now we excite some electrons from valence band to conduction band, after about 1 ps, electrons in the conduction band drop to the lowest unoccupied levels of this band, we name the upper boundary of the electron energy levels in the conduction band the quasi-Fermi level Efc. Meanwhile holes appear in the valence band and electrons near the top of the valence band drop to the lowest energy levels of the unoccupied valence energy levels, leave on the top of the valence band an empty part. We call the new upper boundary energy level of the valence band quasi-Fermi level Efv. When electrons in the conduction band run into the valence band, they will combine with the holes, in the same time they emit photons. This is the recombination radiation. Our task is to make this recombination radiation to laser

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